This disclosure generally relates to blower systems (also referred to as air handling systems) and other fluid handling systems, and more particularly to controls for such systems.
Heating, ventilation and/or air conditioning (“HVAC”) systems commonly have blower systems for moving air. These blower systems typically include a fan (such as a squirrel cage fan), an electric motor for powering the fan, and a control for the electric motor. In some systems, the control receives a signal corresponding to airflow demand from a system controller, such as a thermostat. Variable speed motors are preferred over fixed speed motors because they can be programmed to provide a constant airflow over a range of system static pressures. Static pressure varies in the system due to system changes such as obstructed filters and dust build up. The variable speed motor controls may relay on an equation or a data table to control the speed or torque of the motor to provide generally constant airflow.
A variety of methods is used to characterize a system so that airflow demands can be converted to torque demands. For example, U.S. Pat. Appn. Pub. No. 2007/0248467 A1 describes a method for producing a torque demand from an airflow demand using an equation such as:T=K1+K2*s+K3*A+K4*s*A2,   (1)where T represents the torque demand in Newton meters (Nm), s represents motor speed in revolutions per minute (rpm) and A represents the airflow demand in cubic feet per minute (cfm). K1, K2, K3 and K4 are constants derived for a particular blower system. These constants are derived from torque, speed and airflow data collected for the particular blower system. Other equations (e.g., higher order equations) may be used to calculate airflow demands from torque demands. Regardless of the particular equation used, constants must generally be determined for each system because the constants vary with various system parameters such as the size and make of the fan, fan housing and the system as a whole. The process of determining constants for a system is referred to as system characterization. This type of motor control provides reasonably constant airflow close to the demanded airflow when the motor is within it normal operating window. But when the motor operates in a limit condition, such as a speed limit, a power limit or a temperature limit, the delivered airflow can be much less than the demanded airflow. Smaller than required airflows can reduce the effectiveness of the air handling system, and under some circumstances delivering too little air can damage the system, rendering it inoperable. Knowing the actual airflow may also allow the system controller to forewarn certain system malfunctions. It may also allow the system controller to operate the heating and cooling system in a modified way to avoid over heating or over draft conditions.
In the past, when air handling systems were run in constant airflow mode, the airflow being delivered was assumed to be the demanded airflow. But when the motor was operated at limit conditions such as when the speed limit, torque limit or power limit of the motor was reached, the actual airflow was sometimes much lower than the demanded airflow. When the system was run in constant torque or speed mode of operation, airflow was not estimated. Thus, systems running in constant torque or speed mode did not provide enough airflow as the system static increased. With the airflow estimation, systems running in speed or torque mode can be controlled to maintain a required airflow. Although airflow could be measured using a airflow sensor, using an airflow sensor in duct work is costly and unreliable.